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Project description

Managing the impacts of the introduced European green crab (Carcinus maenas) in coastal estuaries. (02XN021)
Program Exotic Pests and Diseases Research Program
Principal
investigator
E.D. Grosholz, Environmental Science and Policy, UC Davis
Host/habitat Estuary; Marine Area
Pest European Green Crab Carsinus maenas
Discipline Wildlife Biology
Review
panel
Natural Systems
Start year (duration)  2002 (Three Years)
Objectives Determine the consequences of native oyster restoration for introduced European green crab populations.

Determine how experimental reductions of green crab populations will influence the survival of native oysters and the success of oyster restoration.

Determine how reductions in green crabs influence the abundance of native species in oyster restoration plots.

Communicate these results to the local community, shellfish growers, and natural areas managers.

Project
Summary
The proposal focuses on the impacts of the European green crab, which has reduced the abundance of several native species in California estuaries by greater than 90%. Now green crab predators may complicate efforts to restore native oysters in central California. This proposal would investigate whether green crabs will colonize experimental oyster restoration plots and how they may affect the survival of native oysters and other native species associated with restoration plots. I will examine the effectiveness of small-scale reductions of green crabs as a management tool that may reduce the impacts of green crabs on native species.
Final report Our work has provided several tangible benefits to those attempting to enhance and restore populations of the native Olympia oyster (Ostreola conchaphila). Efforts are under way in several bays in California (San Francisco, Tomales Bay, Humboldt Bay) conducted and supported by a range of entities from local recreational groups (Marin Rod and Gun Club) to state (California Ocean Protection Council) and federal agencies (National Marine Fisheries Service).

Our work has helped pioneer many of the techniques that these groups are using and to help quantify the threat that the recently introduced European green crab predators pose to ongoing and future restoration efforts.

We have been able to quantify the extent to which European green crabs in Tomales Bay represent a threat to native oyster restoration. We have experimentally examined the size ranges of oysters that are most vulnerable, as well as the rates of predation that can be expected given a certain density of green crab predators. By using a mixture of field surveys and mensurative and manipulative experiments, we have accomplished the following:

a. We found that European green crabs are a significant predator of juvenile and adult native oysters and that crab size determines prey size.
b. Other predators, including Atlantic oyster drills, were also significant predators of native oysters.
c. Manipulative field experiments with cage exclosures and controls showed that green crabs were the most significant predator.
d. Oyster recruitment is highly variable among years. There was no recruitment at all in either 2003 to 2004 or 2004 to 2005, although very high recruitment in 2002 to 2003.
e. Restoration racks have no more green crabs than natural areas.
f. Size distribution data indicates that recruitment is highest in the back section of the bay and lowest towards the mouth.
g. We found that green crab abundances could be reduced in experimental oyster racks, which could reduce predation on oysters.
h. Experimental reductions of green crabs by trapping resulted in reduced densities for at least two months.

Third-year
progress
We found that European green crabs are significant predators of juvenile and adult native oysters, and that crab size determines prey size. Other predators, including Atlantic oyster drills, were also signficant predators of native oysters.

Manipulative field experiments with cage exclosures and controls showed that green crabs were the most significant predator. Oyster recruitment is highly variable among years. There was no recruitment in either 2003 to 2004 or 2004 to 2005, although very high recruitment in 2002 to 2003. Restoration racks have no more green crabs than natural areas. Size distribution data indicates that recruitment is highest in the back section of the bay and lowest toward the mouth. We found that green crab abundances could be reduced in experimental oyster racks that could reduce predation on oysters. Experimental reductions of green crabs by trapping resulted in reduced densities for at least two months.

Second-year
progress
We found that European green crabs are a significant predator of juvenile and adult native oysters.

We found that green crab abundances could be reduced in experimental oyster racks, which could reduce predation on oysters

Oyster recruitment is highly variable among years with NO recruitment at all in 2003-04, although very high recruitment the previous year.

Oyster recruitment is spatially variable in Tomales Bay with the highest recruitment in the back bay, particularly "hot spots" on the western side with declining recruitment levels towards the outer bay.

First-year
progress
Delays in start date resulted in no work in 2002

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